Make elements optional

[benmccann]

This change would be necessary to allow the fastline controller from #6822 to be implemented externally

[kurkle]

Ok, this is not required through, you could just set meta.data=dataset.data.

I don't actually think this is a good idea

[kurkle]

Edit: I'll have to think this more, it feels like we are going towards more trouble.

[kurkle]

Ok, one issue here is that we can not know if the said controller creates equal amount of elements with the _parsed data, or if it uses _parsed at all.

So I don't think we should make elements optional. _parsed should be optional.

[benmccann]

#6831 adds tests to make sure the arrays stay the same size, so I think it's okay to assume. But the code makes that assumption even without this PR by doing:

	item = metaData[i];
	parsed = meta._parsed[i];

This loop is primarily looping over and doing a calculation on _parsed so it makes most sense to use _parsed.length as the termination condition. We could do ilen = Math.max(meta._parsed.length, metaData.length) if you'd like to loosen the assumption that the two datasets are the same size though

[kurkle]

Fine, we can fix it later if its a problem.